|Publication number||US6928460 B2|
|Application number||US 10/189,054|
|Publication date||Aug 9, 2005|
|Filing date||Jul 1, 2002|
|Priority date||Jul 1, 2002|
|Also published as||US20040003014|
|Publication number||10189054, 189054, US 6928460 B2, US 6928460B2, US-B2-6928460, US6928460 B2, US6928460B2|
|Inventors||Vijay G. Nagarajan, Robert Rochetti, Nedim Fresko|
|Original Assignee||Sun Microsystems, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (39), Non-Patent Citations (10), Referenced by (31), Classifications (9), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
1. Field of the Invention
The present invention relates to managing memory within a computer system. More specifically, the present invention relates to a method and an apparatus for performing generational garbage collection in a segmented heap.
2. Related Art
Modem object-oriented programming systems typically allocate objects from a region of memory known as the “heap”. When an object becomes unreachable because all references to the object have been removed, the object can no longer be used. However, the object continues to occupy memory space within the heap. At a later time, a “garbage collection” process reclaims this unused memory and makes it available to accommodate other objects. This garbage collection process may additionally perform compaction by rearranging objects in the heap to reduce memory fragmentation.
One of the most efficient types of garbage collectors is a “generational garbage collector”. In a generational garbage collector, new objects are allocated in a “young generation” area of the heap. If an object continues to have references over a specified number of garbage collections cycles, the object is promoted to one or more old generation areas of the heap. A generational garbage collector performs garbage collection frequently on the young generation area of the heap, while performing garbage collection less frequently on old generation areas. This tries to match typical program behavior where most newly created objects are short-lived, and are thus reclaimed during garbage collection of the young generation. Long-lived objects in the old generation areas tend to persist in memory. Hence, the old generation areas need to be garbage collected less frequently. This greatly reduces the effort involved in garbage collection because only the young generation area of the heap needs to be garbage collected frequently.
When the system garbage collects young generation area 102, references from old generation area 104 to objects in young generation area 102 need to be located. To make locating these references easier, the system maintains a card table 106 that is associated with old generation area 104. When a pointer in old generation area 104 is updated, a corresponding entry in the card table 106 is marked. For example,
The above-described method of garbage collection works well for an old generation area that is allocated in contiguous memory. A problem exists, however, in using this type of garbage collection technique in small, embedded systems do not support virtual memory. In this type of system, it is not practical to maintain a contiguous heap, which can be grown and shrunk depending on the dynamically changing requirements of applications running on these small systems.
What is needed is a method and an apparatus that provides the advantages of using card tables as described above without having to maintain a large contiguous heap.
One embodiment of the present invention provides a system that facilitates performing generational garbage collection on a heap. The system operates by dividing an old generation of the heap into segments. Next, the system divides each segment into a series of cards and associates a separate card table with each segment. This card table has an entry for each card in the segment.
In a variation on this embodiment, the card table for a given segment is stored within the segment. This facilitates locating the card table.
In a variation on this embodiment, while updating a pointer within an object in the old generation, the system locates the segment containing the object and accesses the card table for the segment. The system then marks the entry in the card table associated with the card containing the object.
In a variation on this embodiment, dividing the old generation of the heap into segments involves dividing the old generation of the heap into segments along power-of-two address boundaries with power-of-two size.
In a variation on this embodiment, the system maintains a hot list of segments that hold objects recently promoted from the young generation to the old generation. The system adds a segment to the hot list when an object in the segment is promoted from the young generation to the old generation.
In a variation of this embodiment, an object that is larger than a normal segment is placed in a large segment by itself. This large segment includes only the large object.
In a variation of this embodiment, the large segment is not moved during heap compaction.
In a variation of this embodiment, the system maintains a list of segments in low-to-high address order to aid memory compaction.
In a variation of this embodiment, while promoting objects from the young generation to the old generation the system promotes the largest objects first. In doing so, the system iterates through segments to find a hole in the old generation that can hold an object from the young generation.
The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.
The data structures and code described in this detailed description are typically stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communications network, such as the Internet.
These segments are linked together from low address to high address in a doubly linked list with links 206 as shown in FIG. 2. Note that other methods of linking these segments together are possible.
Each segment is associated with a card table, such as segment card table 204. By including the card table associated with a segment in the segment, locating and updating the card table can be accomplished with primitive bit operations as described below in conjunction with FIG. 4. Note that maintaining a single monolithic card table for the entire heap would require time-consuming operations to locate entries in the card table, to update the card table, and also waste space covering unused portions of the native heap.
Reference updating mechanism 304 receives object references, which need to be updated. The object references can be to existing objects or to new objects being allocated within young generation area 102. Upon receiving an object reference, reference updating mechanism 304 locates the proper entry in the old generation as described below in conjunction with FIG. 4 and marks the corresponding entry in the card table for the card holding the reference.
Memory allocator 306 accesses the memory management system of the underlying operating system to receive additional segments for the segmented old generation. These segments are typically 64 K-bytes in size and are aligned on 64 K-byte boundaries. However, if an object requires more than 64 K-bytes of memory, a large segment is allocated with sufficient memory to hold the object. This large object is the only object placed in this segment.
Garbage collector 310 operates in a manner similar to other garbage collectors for carded heaps. Garbage collector 310 examines the segment card tables located within the segments to determine cards that have marked entries. When garbage collector 310 promotes an object from the young generation to the segmented old generation, it first attempts to find a place within the old generation to hold the promoted object. If a space cannot be found, memory allocator 306 allocates a new segment and links it into the existing old generation. Note that garbage collector 310 maintains a hot-list (not shown) of segments containing recently promoted objects to facilitate rescanning to update links within these hot-listed segments.
During compaction of the old generation, large segments are not moved. Garbage collector 310 compacts the old generation into as few segments as possible and either releases unused segments to the operating system or maintains a list of unused segments for later use when more memory for allocating objects is required.
Pointer address 402 is typically 32-bits in size. Since the segments are located on 216-byte boundaries, the upper 16-bits of pointer address 402 form segment number 404, while the lower 16 bits form slot location 406 within the segment. Assuming 29-byte card table entries, the upper 7-bits of slot location 406 identify card table index 408. Card table index 408 can be used to directly mark the proper card table entry for the updated card.
Card table address 410 is generated by concatenating segment number 404, nine zeros, and card table index 408. Card table address 410 can be used directly by reference updating mechanism 304 to update the card table entry for the updated card.
Marking Card Table Entries
If space is available at step 606 or after allocating a new segment at step 608, garbage collector 310 places the promoted object into the segment (step 610). Garbage collector 310 then links the segment containing the newly promoted object into the hot list (Step 612).
The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US5845298 *||Apr 23, 1997||Dec 1, 1998||Sun Microsystems, Inc.||Write barrier system and method for trapping garbage collection page boundary crossing pointer stores|
|US5873104 *||Jun 26, 1997||Feb 16, 1999||Sun Microsystems, Inc.||Bounded-pause time garbage collection system and method including write barrier associated with source and target instances of a partially relocated object|
|US5873105 *||Jun 26, 1997||Feb 16, 1999||Sun Microsystems, Inc.||Bounded-pause time garbage collection system and method including write barrier associated with a source instance of a partially relocated object|
|US5900001 *||Apr 23, 1997||May 4, 1999||Sun Microsystems, Inc.||Method and apparatus for optimizing exact garbage collection using a bifurcated data structure|
|US5903900 *||Apr 23, 1997||May 11, 1999||Sun Microsystems, Inc.||Method and apparatus for optimizing exact garbage collection of array nodes in a carded heap|
|US5911144 *||Apr 23, 1997||Jun 8, 1999||Sun Microsystems, Inc.||Method and apparatus for optimizing the assignment of hash values to nodes residing in a garbage collected heap|
|US5915255 *||Apr 23, 1997||Jun 22, 1999||Sun Microsystems, Inc.||Method and apparatus for referencing nodes using links|
|US5920876 *||Apr 23, 1997||Jul 6, 1999||Sun Microsystems, Inc.||Performing exact garbage collection using bitmaps that identify pointer values within objects|
|US5930807 *||Apr 23, 1997||Jul 27, 1999||Sun Microsystems||Apparatus and method for fast filtering read and write barrier operations in garbage collection system|
|US5953736 *||Apr 23, 1997||Sep 14, 1999||Sun Microsystems, Inc.||Write barrier system and method including pointer-specific instruction variant replacement mechanism|
|US6038572 *||Apr 23, 1997||Mar 14, 2000||Sun Microsystems, Inc.||Method and apparatus for localizing nodes in a garbage collected carded heap|
|US6049810 *||Apr 23, 1997||Apr 11, 2000||Sun Microsystems, Inc.||Method and apparatus for implementing a write barrier of a garbage collected heap|
|US6098089 *||Apr 23, 1997||Aug 1, 2000||Sun Microsystems, Inc.||Generation isolation system and method for garbage collection|
|US6115782 *||Apr 23, 1997||Sep 5, 2000||Sun Micosystems, Inc.||Method and apparatus for locating nodes in a carded heap using a card marking structure and a node advance value|
|US6148309 *||Aug 25, 1998||Nov 14, 2000||International Business Machines Corporation||Method for handling overflow of counters in comparison based actions|
|US6148310 *||Aug 25, 1998||Nov 14, 2000||International Business Machines Corporation||Method for combining card marking with remembered sets for old area of a memory heap|
|US6173294 *||Aug 25, 1998||Jan 9, 2001||International Business Machines Corporation||Method for combining card marking with remembered set for generational garbage collection with more than two generations|
|US6185581 *||Aug 19, 1999||Feb 6, 2001||Sun Microsystems, Inc.||Train-algorithm-based garbage collector employing fixed-size remembered sets|
|US6199075 *||May 30, 1997||Mar 6, 2001||Sun Microsystems, Inc.||Method and apparatus for generational garbage collection of a heap memory shared by multiple processors|
|US6226653 *||Jan 10, 2000||May 1, 2001||International Business Machines Corporation||Method and apparatus for performing generational garbage collection using remembered set counter|
|US6249793 *||Jun 10, 1999||Jun 19, 2001||Sun Microsystems, Inc.||Mostly concurrent compaction in a garbage collection system|
|US6308185 *||Feb 9, 1999||Oct 23, 2001||Sun Microsystems, Inc.||Methods and apparatus for generational dynamic management of computer memory|
|US6317756 *||Oct 7, 1998||Nov 13, 2001||International Business Machines Corporation||On-the-fly garbage collector|
|US6415302 *||Aug 19, 1999||Jul 2, 2002||Sun Microsystems, Inc.||Train-algorithm-based garbage collector employing farthest-forward-car indicator|
|US6424977 *||Aug 19, 1999||Jul 23, 2002||Sun Microsystems, Inc.||Train-algorithm-based garbage collector employing reduced oversized-object threshold|
|US6434576 *||Aug 19, 1999||Aug 13, 2002||Sun Microsystems, Inc.||Popular-object handling in a train-algorithm-based garbage collector|
|US6434577 *||Aug 19, 1999||Aug 13, 2002||Sun Microsystems, Inc.||Scalable-remembered-set garbage collection|
|US6449626 *||Aug 19, 1999||Sep 10, 2002||Sun Microsystems, Inc.||Reduced-cost remembered-set processing in a train-algorithm-based garbage collector|
|US6470361 *||Jan 10, 2000||Oct 22, 2002||International Business Machines Corporation||Method and apparatus for performing generational garbage collection using middle-aged objects|
|US6490599 *||Dec 11, 2000||Dec 3, 2002||International Business Machines Corporation||On-the-fly garbage collector|
|US6510440 *||Jan 10, 2000||Jan 21, 2003||International Business Machines Corporation||Method and apparatus for performing generational garbage collection using barrier bits|
|US6526422 *||Oct 31, 2000||Feb 25, 2003||Sun Microsystems, Inc.||Striding-type generation scanning for parallel garbage collection|
|US6529919 *||Feb 15, 2000||Mar 4, 2003||Sun Microsystems, Inc.||Incremental class unloading in a train-algorithm-based garbage collector|
|US6560619 *||Nov 1, 2000||May 6, 2003||Sun Microsystems, Inc.||Using atomic compare-and-swap operations for forwarding-pointer installation|
|US6823351 *||Oct 26, 2000||Nov 23, 2004||Sun Microsystems, Inc.||Work-stealing queues for parallel garbage collection|
|US6826583 *||Oct 25, 2000||Nov 30, 2004||Sun Microsystems, Inc.||Local allocation buffers for parallel garbage collection|
|US6845437 *||Aug 13, 2002||Jan 18, 2005||International Business Machines Corporation||Computer system with heap and card table|
|US20020161792 *||Jun 20, 2002||Oct 31, 2002||Garthwaite Alexander T.||Garbage collector employing multiple-car collection sets|
|US20030033498 *||Aug 13, 2002||Feb 13, 2003||International Business Machines Corporation||Computer system with heap and card table|
|1||*||Azagury, A., E.K. Kolodner, E. Petrank and Z. Yehudai "Combining Card Marking with Remembered Sets: How to Save Scanning Time", ACM SIGPLAN Notices, vol. 34, No. 3, 1999, pp. 10-19.|
|2||*||Borman, S. et al. "A Serially Reusable Java(TM) Virtual Machine Implementation for High Volume, Highly Reliable Transaction Processing", Technical Report TR29.3406, IBM, downloaded from www-1.ibm.com/servers/eserver/zseries/software/java/pdf/29.3406.pdf,2000.|
|3||*||Flood, C.H. and D. Detlefs "Parallel Garbage Collection for Shared Memory Processors", Proceedings of the USENIX Java(TM) Virtual Machine Research and Technology Symposium (JVM '01), Apr. 23-24, 2001.|
|4||*||Hölzle "A Fast Write Barrier for Generational Garbage Collectors", ACM Conference on Object-Oriented Programming System Languages and Applications (OOPSLA '93), 1993.|
|5||*||Hosking, A.L. and R.L. Hudson "Remembered Sets Can Also Play Cards", ACM Conference on Object-Oriented Programming Systems Languages and Applications (OOPSLA '93), 1993.|
|6||*||Hosking, A.L., J.E.B. Moss and D. Stefanovic "A Comparative Performance Evaluation of Write Barrier Implementation", ACM Conference on Object-Oriented Programming Systems Languages and Applications (OOPSLA '92), 1992, p. 92-109.|
|7||*||Jones, R. and R. Lins "Garbage Collection: Algorithms for Automatic Dynamic Memory Management", Chichester:John Wiley & Sons, 1996, Chapter 7, pp. 143-181. ISBN 0-471-94148-4. QA76.9.G37J66 1996.|
|8||*||Printezis, T. and D. Detlefs "A Generationsl Mostly-Concurrent Garbage Collector", Proceedings of the 2<SUP>nd </SUP>International Symposium on Memory Management, Oct. 15-16, 2000, pp. 143-154.|
|9||*||Seligmann, J. and S. Grarup "Incremental Mature Garbage Collection Using the Train Algorithm", Proceedings of the 9th European Conference on Object-Oriented Programming (ECOOP '95), Aug. 1995, pp. 235-252.|
|10||*||Wilson, P.R. and T.G. Moher "A 'Card-Marking' Scheme for Controlling Intergerational References in Generation-Based Garbage Collection on Stock Hardware", ACM SIGPLAN Notices, vol. 24, No. 5, May 1989, pp. 87-92.|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US7085790||Dec 6, 2002||Aug 1, 2006||Sun Microsystems, Inc.||Advancing cars in trains managed by a collector based on the train algorithm|
|US7096238 *||Dec 6, 2002||Aug 22, 2006||Sun Microsystems, Inc.||Dynamic feedback for determining collection-set size|
|US7127709 *||Sep 25, 2002||Oct 24, 2006||Microsoft Corporation||System and method for jointly managing dynamically generated code and data|
|US7146390||Feb 24, 2003||Dec 5, 2006||Sun Microsystems, Inc.||Staging the processing of remembered-set entries as part of collection based on the train algorithm|
|US7188129||Nov 15, 2002||Mar 6, 2007||Sun Microsystems, Inc.||Merging trains in a collector based on the train algorithm|
|US7209935||Nov 27, 2002||Apr 24, 2007||Sun Microsystems, Inc.||Avoiding remembered-set maintenance overhead for memory segments known to be in a collection set|
|US7284108||Sep 23, 2005||Oct 16, 2007||Microsoft Corporation||Systems and methods for multiprocessor scalable write barrier|
|US7321909||Dec 23, 2004||Jan 22, 2008||Sun Microsystems, Inc.||Method and apparatus for forwarding references to objects concurrently with space-incremental garbage collection|
|US7404182||Oct 3, 2003||Jul 22, 2008||Sun Microsystems, Inc.||Deferring and combining write barriers for a garbage-collected heap|
|US7584231 *||May 25, 2006||Sep 1, 2009||Sun Microsystems, Inc.||Methods for determining a safe end of scan for generational garbage collection|
|US7599973 *||Jan 12, 2006||Oct 6, 2009||Sun Microsystems, Inc.||Method and apparatus for decreasing object copying by a generational, copying garbage collector|
|US7627621 *||Feb 12, 2007||Dec 1, 2009||Sun Microsystems, Inc.||Method and system for minor garbage collection|
|US7650371 *||Dec 14, 2006||Jan 19, 2010||Microsoft Corporation||Finalizable object usage in software transactions|
|US7664800||Jan 22, 2007||Feb 16, 2010||Samsung Electronics Co., Ltd.||Apparatus and method for managing resource on java environment|
|US7716449||Jul 12, 2007||May 11, 2010||Oracle America, Inc.||Efficient chunked java object heaps|
|US7730278||Jul 12, 2007||Jun 1, 2010||Oracle America, Inc.||Chunk-specific executable code for chunked java object heaps|
|US7870171||Feb 12, 2007||Jan 11, 2011||Oracle America, Inc.||Method and system for garbage collection in a multitasking environment|
|US8131955 *||Apr 15, 2004||Mar 6, 2012||Microsoft Corporation||Ephemeral garbage collection using a tracking mechanism on a card table to determine marked bundles|
|US20040060041 *||Sep 25, 2002||Mar 25, 2004||Microsoft Corporation||System and method for jointly managing dynamically generated code and data|
|US20040111448 *||Dec 6, 2002||Jun 10, 2004||Garthwaite Alexander T.||Dynamic feedback for determining collection-set size|
|US20050235120 *||Apr 15, 2004||Oct 20, 2005||Microsoft Corporation||System and method for performing garbage collection on a large heap|
|US20060020766 *||Sep 23, 2005||Jan 26, 2006||Microsoft Corporation||Systems and methods for multiprocessor scalable write barrier|
|US20070100919 *||Oct 20, 2006||May 3, 2007||Electronics And Telecommunications Research Institute||Garbage collection unit and method thereof|
|US20070174370 *||Jan 12, 2006||Jul 26, 2007||Sun Microsystems, Inc.||Method and apparatus for decreasing object copying by a generational, copying garbage collector|
|US20080147757 *||Dec 14, 2006||Jun 19, 2008||Microsoft Corporation||Finalizable object usage in software transactions|
|US20080195680 *||Feb 12, 2007||Aug 14, 2008||Sun Microsystems, Inc.||Method and system for minor garbage collection|
|US20080195681 *||Feb 12, 2007||Aug 14, 2008||Sun Microsystems, Inc.||Method and system for garbage collection in a multitasking environment|
|US20090019221 *||Jul 12, 2007||Jan 15, 2009||Kessler Peter B||Efficient chunked java object heaps|
|US20090019249 *||Jul 12, 2007||Jan 15, 2009||Kessler Peter B||Chunk-specific executable code for chunked java object heaps|
|US20100293206 *||May 12, 2009||Nov 18, 2010||Tatu Ylonen Oy Ltd||Clustering related objects during garbage collection|
|CN104063245A *||Jun 24, 2014||Sep 24, 2014||用友优普信息技术有限公司||Memory analysis and optimization method and system based on garbage collection|
|U.S. Classification||1/1, 711/E12.012, 707/999.206|
|International Classification||G06F17/30, G06F12/00, G06F12/02|
|Cooperative Classification||Y10S707/99957, G06F12/0276|
|Jul 1, 2002||AS||Assignment|
Owner name: SUN MICROSYSTEMS, INC., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:NAGARAJAN, VIJAY G.;ROCHETTI, ROBERT;FRESKO, NEDIM;REEL/FRAME:013088/0588
Effective date: 20020603
|Jan 7, 2009||FPAY||Fee payment|
Year of fee payment: 4
|Jan 9, 2013||FPAY||Fee payment|
Year of fee payment: 8
|Dec 12, 2015||AS||Assignment|
Owner name: ORACLE AMERICA, INC., CALIFORNIA
Free format text: MERGER AND CHANGE OF NAME;ASSIGNORS:ORACLE USA, INC.;SUN MICROSYSTEMS, INC.;ORACLE AMERICA, INC.;REEL/FRAME:037278/0853
Effective date: 20100212
|Jan 26, 2017||FPAY||Fee payment|
Year of fee payment: 12